This invention relates generally to transformer internal fault protection for electric power transformers, and more specifically concerns differential protection for such a transformer which includes harmonic restraint and blocking capability.
It is important to first recognize and then take corrective action in response to the occurrence of internal faults in a power transformer. Prompt disconnection of the faulty transformer from the power system is necessary to avoid possible extensive damage to the transformer as well as to preserve power system stability and power quality.
Several different techniques have been developed to detect internal transformer faults, which initially included phase overcurrent protection, differential protection using differential currents, and gas accumulator/rate of pressure rise protective techniques for arcing faults. Differential protection is currently widely used as protection against relay internal faults. In differential protection, operating current (IOP) (which may also be referred to as differential current), is compared with a restraining current (IRT ) The operating current is defined as the phasor sum of currents entering the protected element (the transformer) as shown in the following equation and in FIG. 1.
IOP=|{right arrow over (IW1+L )}+{right arrow over (IW2+L )}|
In FIG. 1, the power transformer 10 is protected by differential relay 11. CT1 and CT2 are current transformers. The operating current is proportional to the fault current for transformer internal faults and approaches zero for non-faulted conditions. There are several equations for restraining current, including the following equation:
IRT=k|{right arrow over (IW1+L )}xe2x88x92{right arrow over (IW2+L )}|
where k is a compensation factor, usually 1 or 0.5. A trip signal is generated if the operating current is greater than a selected percentage of the restraining current.
Conventional differential protection arrangements may misoperate, however, in the presence of what are known as transformer inrush currents, which are the result of transients in the magnetic flux within the transformer.
It has been recognized that the harmonic content of the differential current provides information which can help to differentiate actual internal faults from an inrush condition. Restraining techniques have been used which utilize all the harmonics of the differential current to distinguish inrush currents from internal fault conditions, so as to restrain a tripping action which would otherwise occur when there is in fact no internal fault.
As indicated above, known differential restraining protection typically uses all of the harmonics, or a combination of selected odd and even harmonics, of the differential current. A blocking arrangement, which results in blocking of the trip signal, has also been used. In one blocking arrangement, a second harmonic of the differential current is used. Existing transformer differential relays thus use either all-harmonic restraining or blocking techniques to distinguish between internal faults and inrush conditions. In addition, a fifth harmonic restraining technique has been used to prevent misoperation in response to transformer over-excitation, as opposed to internal faults.
These restraining and/or blocking techniques have resulted in increased security relative to inrush and transformer over-excitation, but can result in a delay of tripping action for actual internal faults under certain conditions.
Other techniques, such as recognition of wave shape distortion and recognition of the time interval during which the differential current is near zero, have been used. Still other techniques include recognition of DC offset or asymmetry in the differential current and the comparison of the amplitudes of positive and negative thresholds of differential current, with the selected thresholds being in two different polarized elements. These techniques are often used as enhancements to the basic restraining and/or blocking functions described above. However, no combination of these techniques have been found which provide an adequate solution, i.e. security for several specific inrush conditions.
Hence, there remains an opportunity for significant improvement in distinguishing internal faults from inrush and over-excitation conditions for particular power system operating conditions.
Accordingly, the present invention is for power transformer differential protection and comprises: means for developing a differential current value from processed current values which are representative of transformer (CT) secondary current values obtained from the windings of a power transformer; means for developing an operating current value from said differential current value; means for developing a restraining current value from said processed current values; means for obtaining even harmonic values of said differential current value; and means for comparing the sum of the restraining current values and the even harmonic values with the operating current value and for producing an output signal which in turn is useful in producing a trip signal when the operating current value is larger than said sum.